In recent years, the measurement of magnetic signals generated by the physiological activity of various organs of the human body has gained increasing interest. Such measurements have been made possible by the development of superconducting instrumentation having sufficient sensitivity. The magnetic detector so utilized is a superconducting magnetometer or "SQUID". These detectors have an extremely high sensitivity of up to 10.sup.-4 Tesla/ .sqroot.-Hz in the frequency range D.C.+20 KHz. On the other hand, neuromagnetic fields are extremely weak from an amplitude of 10.sup.-12 T, for the higher signals due to spontaneous activity, to an amplitude of less than 10.sup.-13 Tesla for the inducted neuromagnetic signals. These values are many magnitude orders lower than both the earth magnetic field and than urban magnetic disturbances substantially due to the displacement of large ferromagnetic masses (elevators, machines, etc.) and than the magnetic fields generated by power mains.
Therefore, the problem is to measure extremely weak signals in environments in which the background magnetic noise is of many orders higher than the signals themselves.
To extract the signal to be measured from such high background noise, one can either operate in magnetically shielded environments, or perform a spatial rejection by means of a 2nd derivative gradiometer.